Just like language translation, color conversion is not always a straight-forward one-to-one mapping. Specifically, RGB has less degrees of freedom (3) than CMYK (4). Therefore, although one can convert any RGB to CMYK without losing information, the inverse is not the case.
When a CMYK color is converted to RGB, some information is lost. When you convert that RGB color back to CMYK, you will in most cases get a different (but similar) color.